Voltage regulators and other power management circuits are routinely used in a wide variety of electronic devices. Over-current protection is often a critical function of these power management circuits. Over-current protection typically helps to protect electronic circuitry from excessive current during short-circuit conditions, which can interfere with, damage, or destroy the electronic circuitry.
A conventional low drop out (LDO) regulator over-current protection circuit 100 is shown in FIG. 1. The circuit 100 includes an error amplifier 102, p-channel metal oxide semiconductor (PMOS) transistors 104-112, n-channel metal oxide semiconductor (NMOS) transistors 114-120, and resistors 122-126. The error amplifier 102 receives two input voltages, a reference voltage VREF and a feedback voltage VFB. The error amplifier 102 also generates output signals based on any differences between the voltages VREF and VFB.
The transistor 106 represents a pass device, and the transistor 110 represents a sense device. The transistors 104 and 114 form a driver that drives the transistors 106 and 110. The transistor 106 generates an output voltage VOUT. The transistor 110 produces a sense current, which is provided to the transistor 118 and mirrored by the transistor 120. A bias signal BIAS_P is provided to the gate of the transistor 112. A voltage at the drain of the transistor 112 is provided to the gates of the transistors 108 and 116, which function as switches. The resistors 124-126 form a voltage divider that generates the feedback voltage VFB.
During normal operation, the sense current flowing through the transistor 110 is provided to the transistor 118, and the transistor 120 mirrors the sense current. Also, a bias current flows through the transistor 112, which turns off the transistor 108 and turns on the transistor 116.
During a short-circuit condition (when the output current becomes too high), the current flowing through the transistors 118-120 is greater than the bias current of the transistor 112. This pulls the gate of the transistor 108 down and turns on the transistor 108. As a result, this pulls up the gates of the transistors 106 and 110, thereby limiting the current flowing through the transistors 106 and 110.
In this short-circuit condition, the output current is clamped at a short-circuit current limit, which could be approximately 1.5 to 3 times the maximum load current. Also, the output voltage may drop to below 0.5V. Because of this, the power loss and thermal generation in the circuit 100 could be significantly high, which may increase the risk of thermal-induced device failure.